Topographic Steering of the Upper Arctic Ocean Circulation by Deep Flows

Dynamically, the Arctic Ocean is characterised by the presence of closed f/H contours, where f is the Coriolis parameter and H the depth. On closed f/H contours, a net integrated surface wind stress can theoretically drive relatively strong near-bottom flows. Nevertheless, the Rossby number of the large-scale time-mean flow in the Arctic Ocean is estimated to be small, implying that the near-bottom flow should essentially be aligned with the f/H contours. Observations indicate that the time-mean surface flow also tends to follow the f/H contours, which in the Arctic are essentially controlled by H. To examine mechanisms that can organise the Arctic Ocean surface flow along the topography, we use a two-layer large-scale geostrophic model on an f-plane (exploiting that f/H variations are dominated by depth variations). The effect of time-dependent baroclinic eddies is represented as an eddy diffusion of the upper-layer thickness. We study how wind forcing, stratification, eddy diffusivity and bottom friction affect the topographic steering of the time-mean surface flow, introducing relevant non-dimensional parameters. The analyses suggest that the Arctic Ocean is in a regime where strong along-isobath near-bottom flows can align the buoyancy field and, thereby, the surface currents with the topography. We then discuss the model results in relation to satellite-derived surface currents in the Arctic Ocean and briefly consider additional mechanisms that can align surface flows with the topography.